Subgenotype A1 of HBV--tracing human migrations in and out of Africa.

BACKGROUND: HBV subgenotype A1 is the dominant genotype A strain in Africa, with molecular characteristics differentiating it from A2, which prevails elsewhere. Outside Africa, A1 is confined to areas with migration history from Africa, including India and Latin America. The aim of this study was to reconstruct A1 phylogeny on a spatial scale in order to determine whether A1 can be used to track human migrations.
METHODS: A phylogenetic comparison of A1 was established using neighbour-joining analysis of complete genomes, and the Bayesian method, implemented in BEAST, was performed on the S region of isolates from 22 countries. Migration events were estimated by ancestral state reconstruction using the criterion of parsimony.
RESULTS: From the tree reconstruction, nucleotide divergence calculations and migration analysis, it was evident that Africa was the source of dispersal of A1 globally, and its dispersal to Asia and Latin America occurred at a similar time period. Strains from South Africa were the most divergent, clustering in both the African and Asian/American clades and a South African subclade was the origin of A1. The effect of the 9th to 19th century trade and slave routes on the dispersal of A1 was evident and certain unexpected findings, such as the co-clustering of Somalian and Latin American strains, and the dispersal of A1 from India to Haiti, correlated with historical evidence.
CONCLUSIONS: Phylogeographic analyses of subgenotype A1 can be used to trace human migrations in and out of Africa and the plausible sites of origin and migration routes are presented.